Typically, kiln plants for manufacturing pulverous or granular materials such as cement consist of one or more preheaters for preheating the raw materials supplied, a kiln for burning and sintering the materials, and a cooler for cooling the product treated in the kiln. The preheating takes place by means of the hot kiln exit gases and/or hot spent cooling air which is passed up through the preheater or preheaters.
In order to reduce the dimensions of such a plant while at the same time achieving an increase in its throughput, a kiln plant may be provided in addition with at least one precalciner for precalcining (i.e., expulsion of CO.sub.2) the preheated raw materials prior to passage of these materials to the kiln for the final burning and sintering. Supply of combustion air to such a precalciner is effected by supplying exit gases from the kiln and/or spent cooling air from the cooler as well as by adding a desired amount of atmospheric air to obtain the degree of combustion desired. The precalciner is fed directly with fuel which is mixed with the pulverised raw material in the precalcining zone and burned together with the latter. Subsequently, the precalcined raw materials are passed onto the kiln. In certain kinds of plants, the precalcining does not take place in a separate precalciner, but is arranged to take place in the riser pipe between the kiln and the lowermost preheater stage, the riser pipe being suitably designed for this purpose.
If the fuel used in the precalciner is oil or natural gas, the mixing of the fuel with the pulverised raw material in the precalcining zone does not give rise to major problems in obtaining optimum combustion conditions. However, because of the desire to conserve oil and natural gas resources, it is becoming increasingly popular to use raw coal or other solid fuels for firing in the precalcining zone. In such cases the coal must be crushed prior to the firing in order to achieve the best possible utilization of the coal. Until now, dried and preground coal has normally been used for this purpose although it is also known to feed unground and dried coal into the precalcining zone. In both cases, however, large agglomerations of coal or lumps of coal have been found to be present and to pass the precalcining zone unburned. Thereafter, such agglomerations or lumps fall down through the riser pipe to the kiln proper or end up in the hot air duct through which spent cooling air for the calcining combustion is supplied. Thus, the previous methods have not fully utilized the solid coal in the precalcining zone and have also given rise to problems of clogging both during the pregrinding and during the conveyance of the fuel to the precalcining zone.